In telecommunication networks, communicating nodes may need to have a common time base in order to be able to properly communicate. Thereto, synchronization between different clocks may be performed between various nodes of these networks.
Specifically, base stations of a radio access network are synchronized to a time or frequency reference. Any clock of any device that can provide accurate frequency and/or time may serve as reference clock. The reference clock can be another network node, a separated time server (NTP, PTP), a GPS receiver.
One protocol used is the so-called Network Time Protocol, NTP, specified by the Internet Engineering Task Force, IETF, and described in a suite of specification documents, such as rfc 5905, rfc 5906, rfc 5907, and rfc 5908. Another protocol is the so-called Precision Clock Synchronization Protocol of IEEE, e,g, described in the document IEEE 1588, version 2, titled “IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems”
NTP offers different synchronization modes. In one of these modes, an NTP client synchronizes its clock to a remote NTP server. Therein, clock synchronization is a continuous process, resulting in a sequence of measurements resulting in a clock adjustment. Thereto, the client may repeatedly calculate a round-trip delay time and an offset with respect to the server. These values may be subject to processing and filtering. The client clock is then adjusted to steadily reduce the offset.
After a start or restart of a base station, the base station clock, in the following also being referred to as client clock, is in a so-called free running state, until the client clock meets certain (predefined or specified) accuracy requirements. Thereto, the base station may comprise a synchronization control module to decide when the clock meets the specified accuracy requirements. The synchronization control module thereto may receive clock estimates from an estimation filter, e.g. a so-called Kalman filter, and may determine a comparison curve indicative of a degree of deviation between the reference and the client clock that can be compared with a threshold. When the comparison curve is below the threshold, the synchronization control module may declare the state to be locked. Otherwise the synchronization control module may declare the state to be free-running.
As soon as the accuracy requirements are met, the clock is regarded as synchronized to the reference and the node may start to handle the traffic. As long as the clock is in the free running state, the base station may not handle radio network data traffic.
Measurements of the time difference between a client clock and a reference are subject to noise. Low-noise measurements may arise with GPS as reference, where quantization noise is typically a few nanoseconds, and the GPS receiver gives a few tens of nanoseconds. High-noise measurements may arise when timing information is transferred through a packet switching network, where a packet delay variation, PDV, may add a noise of microseconds, or milliseconds, or even more.
Due to the noise, the comparison curve may vary around the threshold such that the clock's state may toggle between free running and locked mode until it finally settles in the locked mode. In case of high noise, it may happen that the clock's state toggles for a quite long period of time and in a worst case may never settle in the locked mode.
The consequence of the toggling is that the network node that already has started handling traffic may have to be stopped from handling the traffic. E.g. if the network node is a radio base station, already established calls may be disconnected. Disconnecting already established calls may be however worse than a later start of traffic handling e.g. if neighbor nodes are capable of handling the traffic without interrupts until the toggling radio base station is finally synchronized.